The present invention relates generally to the field of devices for reducing interference to a desired signal caused by interfering signal sources. More particularly, the present invention relates to radio frequency apparatus and to those devices which are used to cancel unwanted, interfering signals that may appear in a transmission link between an antenna and a receiver.
There are a number of techniques which have been developed to reduce the effects of interference which may appear on a transmission line which also carries a desired signal. For instance, if the desired signal and the interference are separated in frequency, filters may be used which pass the desired signal and suppress the interference. However, if a filter is used, frequency components of the desired signal which fall within the rejection band will also be suppressed. Filters can introduce significant attenuation even for the desired signal. A filter can also introduce significant absolute and differential group delay times. It is known, for instance, that the absolute group delay time through a band reject filter is approximately the inverse of the pass bandwidth of the filter. For example, the absolute delay time through a 5 MHz bandwidth reject filter will be about 200 nanoseconds.
A further alternative to cancelling interfering signals is the technique of blanking the input to the receiver during the time the interference is present. With this technique, however, no signal can be received during the blanking period. Therefore, this technique is only effective if the blanking does not eliminate a significant fraction of the desired signal. Therefore blanking cannot be used if the interfering signal is continuous or has a high duty cycle. Furthermore blanking requires either a hard-wired connection to the interfering signal or some other technique e.g., predictive analysis, to derive timing information for blanking activation.
Another alternative is that of combining blanking with the use of band reject filters. In this technique, the filter is switched in only during the time the interference is present. However, utilization of this technique still results in significant time-varying group delay propagation times through the filter during the time period in which the filter is switched in and out and also still introduces time-varying differential group delay time.
A still further alternative solution to the problem of cancelling interference is the use of a signal canceller. A prior art signal canceller system is illustrated by way of example in FIG. 1. As is illustrated in FIG. 1, an interfering signal is generated by interfering transmitter 12 and is propagated along transmission line 14 to antenna A.sub.i which radiates the interfering signal. Antenna A.sub.s receives a desired signal and this desired signal is propagated along transmission line 16 to signal receiver 18. Because of coupling from A.sub.i to A.sub.s the signal receiver receives an admixture of both the desired and interfering signals. The interfering signal level S.sub.i, at the output of A.sub.s, is: EQU S.sub.i (t)=Gs.sub.i (t-t.sub.i) (1),
where
S.sub.i =amplitude of interfering signal at input to A.sub.i ; PA1 G=the coupling coefficient from DC1 to DC2 via the A.sub.i -A.sub.s antenna coupling path; G is a complex coefficient specifying both the amplitude and phase of the coupling coefficient; PA1 t.sub.i =group delay time from DC1 to DC2 via the A.sub.i -A.sub.s antenna coupling path. PA1 s.sub.i =amplitude of interfering signal at input to A.sub.i ; PA1 g=the coupling coefficient from DC1 to DC2 via the interference injection coupling path; g is a complex coefficient specifying both the amplitude and phase of the coupling coefficient; PA1 tj=group delay time from DC1 to DC2 via the interference injection coupling path. PA1 t'=t-ti; PA1 td=t.sub.j -t.sub.i, the difference in group delay propagation time between the antenna coupling path and the injection path.
In order to cancel the interfering signal, S.sub.i, a sample of the interfering signal taken at directional coupler DC1 is injected through directional coupler DC2 into the receiver transmission line 16 at a signal level S.sub.j. The level of cancelling signal S.sub.j is: EQU S.sub.j (t)=gs.sub.i (t-t.sub.j) (2),
where
The resultant sum will be: EQU S.sub.i (t)+S.sub.j (t)=Gs.sub.i (t')+gs.sub.i (t'-td) (3),
where
As can be seen from equations (1), (2), and (3) above, for perfect cancellation g=-G and td=0. In practice there will be errors, possibly time-varying errors which will result in a residual interfering signal which can be sampled at coupler DC3. The residual signal must be measured and refinements made in the amplitude, phase and group delay propagation time of the injected signal to further reduce the amplitude of the residual signal. Thus the canceller utilizes a null-seeking feed-back loop. Analog implementations of the feed-back loop to control the amplitude and phase of the injected signal have been utilized for a number of canceller applications. However, hard-wired analog control of the feed-back loop is unsatisfactory or difficult to implement, and is inflexible in any case, for many system configurations, particularly those in which there is low-duty cycle frequency-hopping and/or wide-band interference, variations in coupling G or group delay t.sub.d e.g., mechanical or electrical rotation of A.sub.i, and/or A.sub.s, and a wide variety of other complex system configurations and modes.